It is well known that chemical reactions which proceed slowly at low temperatures can be accelerated greatly by increasing the temperature of the reactants. Thus, many large-scale, high-temperature reactions are carried out daily in a variety of industrial applications. In many instances, it is necessary to conduct these high-temperature reactions at pressures substantially greater than atmospheric pressure. In the past this has been achieved primarily, through the use of massive, thick-walled, high pressure, above-ground reactors having complex mechanical stirring mechanisms. A reaction mixture typically is injected into such an above-ground reactor using a high-pressure pump and the reactants are then heated to bring about an accelerated chemical reaction. When the reaction is complete, the reaction products are removed from the reactor and the process is repeated. For the most part, these above-ground reactors are expensive to construct, maintain and operate.
As a desirable alternative to above-ground reactors, attempts have been made to design low-profile, subterranean or "down-hole" reaction apparatus. The concept of a below-ground reaction apparatus is, of course, appealing from the stand-point of land usage. Other significant advantages are, however, also attained by subsurface construction. In particular, a vertical, down-hole reaction apparatus can now be built which utilizes gravity and thermodynamics to provide a high-pressure reaction environment in which thermal energy is conserved. This remarkable downhole reaction apparatus is disclosed in U.S. Pat. No. 4,272,383 to J. L. McGrew which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference.
It has been found that the McGrew apparatus is especially effective in the destruction of municipal waste by aqueous-phase or "wet" oxidation. Aqueous-phase oxidation of combustible matter is an exothermic reaction which proceeds quite rapidly at temperatures above 350.degree. F. The wet oxidation of municipal waste produces a low-volume, sterile ash, a liquid effluent and off-gases, portions of which may be reclaimed for industrial and agricultural use. Most importantly, wet oxidation in the McGrew apparatus efficiently and substantially reduces the oxygen demand of the waste which is of major concern in the discharge of treated wastes into receiving waters.
Generally, the McGrew down-hole, wet-oxidation reaction apparatus includes a vertical assembly of pipes or tubes which are suspended in a cased well. The pipes extend approximately 5000 feet below ground level and are arranged concentrically to define a series of annuli. The assembly has a central bore which serves as the downgoing passage of a heat exchanger. The first or innermost annulus is closed at its lower end in flow communication with the downgoing passage of the heat exchanger. This annulus functions as the upcoming passage of the central, concentric, heat exchanger. A heat-transfer medium such as oil is circulated through the heat exchanger by pumping it into the downgoing passage and then flowing it back up through the upcoming passage. The heat exchanger is significant not only because energy conservation is a matter of great importance, but also because it functions to regulate the temperature of the reactants. Thus, as will be explained more fully, the heat exchanger of a vertical, down-hole, wet-oxidation reaction apparatus is used to control the rate of reaction of the reactants by selectively supplying or removing heat.
In the McGrew apparatus, the tubes which define the downgoing passage and upcoming passage or annulus of the heat exchanger are positioned in the bore of a somewhat larger pipe or tube such that a second annulus is defined. This second annulus is the downgoing or influent passage for the reactants. However, the volume of the downgoing passage is significantly limited by the presence of the centrally disposed heat exchanger which must be large enough to achieve rate-controlling heat-transfer. A third annulus which is the upcoming or effluent passage is formed by an outer tube which surrounds the tube enclosing the second annulus. This outer tube is capped at its lower end such that the upcoming passage is in flow communication with the downgoing passage.
In operation, a reactant mixture is flowed into the downgoing reactant passage, which, as stated, is in heat exchange relation to the heat exchanger. In the case of municipal waste destruction, for which the McGrew apparatus is particularly well-suited, the reaction mixture includes diluted municipal waste having a chemical oxygen demand of from about 1.0 to 6.0 percent. As the diluted waste is pumped into the downgoing reaction passage, heat is supplied by the centrally disposed heat exchanger. This is achieved by pumping a heat-transfer medium through an above-ground heater and then through the flow pasasges of the heat exchanger, the annulus of which is adjacent the downgoing reactant passage. In addition to the diluted municipal waste, gaseous oxygen, alone, or present in a mixture of gases, is also injected through gas supply lines suspended in the downgoing reactant passage. The flow rate of the diluted municipal waste and the gaseous oxygen are regulated to provide a mixed flow velocity or flow pattern which promotes intense mixing to enhance mass transfer between the available oxygen and the combustible components of the municipal waste.
As the concentration of available oxygen and the temperature of the waste increase, the rate of the wet oxidation reaction increases. The exothermic oxidation reaction generates substantial heat which, in turn, further elevates the temperature of the reactants. When the temperature of the reactants exceeds about 350.degree. F. to 400.degree. F., the reaction becomes autogenous and it is no longer necessary to supply heat to the system. The fluid pressure exerted by the hydrostatic head of the approximately mile long column of diluted waste prevents the high temperature reaction mixture from boiling. In order to optimize the aqueous-phase oxidation reaction, the temperature of the reaction mixture is allowed to rise to about 500.degree. F. to 550.degree. F. in a reaction zone in the lower part of the down-going reactant passage where the reaction temperature is then maintained by removing heat with the heat exchanger. Thus, the heat exchanger provides heat to the reactants during start-up and helps regulate the reaction during continuous operation. Excess thermal energy produced by the reaction can be converted to electrical energy or the like simply by circulating the heat-transfer medium through, for example, a steam turbine.
The McGrew apparatus is preferably operated as a continuous-flow device. The diluted waste is substantially oxidized at elevated temperatures and pressures as it moves through the reaction zone. At the bottom of the reaction apparatus the reaction products or effluent are flowed into the upcoming passage to ground level and removed for further treatment or disposal.
Other, less efficient, down-hole reactors are known. For example in U.S. Pat. No. 3,449,247 to Bauer, a down-hole reaction apparatus is disclosed which does not include a centrally disposed heat exchanger. In the Bauer Patent no means is set forth or suggested by which the temperature of the reactants in the reaction zone can be controlled. In U.S. Pat. No. 3,606,999 to Lawless, disclosing another down-hole process, excess heat generated by exothermic reactions is removed and re-used with a complicated vapor collection procedure. It is also suggested by Lawless that the rate of reaction can be decreased by diluting the waste feed to lower the concentration of combustible matter. Finally, in the down-hole apparatus disclosed in U.S. Pat. No. 3,853,759 to Titmus, a steam line centrally disposed in the bore of the upcoming waste passage is used to heat the effluent which is in heat-transfer relation to the downgoing reactants. It is suggested that the reactant be re-circulated through the system during start-up to achieve operating temperatures. None of these other devices provide the advantages of the McGrew heat exchanger design.
The down-hole reaction apparatus initially proposed by McGrew provides a highly efficient device for the wet oxidation of municipal waste. An improved apparatus utilizing the principles of McGrew was successfully operated experimentally in Longmont, Colo., processing about 120 gallons of diluted waste per minute. It would be desirable to treat even larger volumes of waste using the principles of the McGrew device. However, in order to process larger amounts of waste with a single reaction apparatus, the waste flow passages must somehow be enlarged. This could be achieved in the McGrew-like apparatus by enlarging the internal diameters of the tubes which define the influent and effluent passages. Alternatively, or in addition to enlarging the foregoing tubes, the centrally disposed heat exchanger could be made smaller by decreasing the diameter of the tubes from which it is formed. Both of these alternatives, however, suffer from serious drawbacks.
Increasing the size of the reaction tubes is expensive and requires a correspondingly larger cased well. Since the cost of materials used to form the reaction tubes varies widely depending on availability and composition, increases in tube size may be quite cost intensive. While decreasing the size of the centrally disposed heat exchanger creates more space for the reactants, it also significantly limits the heat exchange capacity of the heat exchanger due to a reduction in both volume and surface area. In order to increase the volume of waste which could be contained in the reaction apparatus in such a manner that the reaction temperature could still be regulated by the centrally disposed heat exchanger, all of the tubes or "stringers", including those comprising the heat exchanger, had to be enlarged. Therefore, it would be highly desirable to provide a vertical, down-hole reaction apparatus which can process a large volume of reactants and by which precise control of reaction temperature can be attained. The present invention provides such an apparatus and a method of operating the novel apparatus which is especially suitable for the large-scale destruction of municipal waste.